JP4684354B2 - Filmy graphite - Google Patents

Description

  The present invention relates to a film-like graphite used as a heat dissipation film, a heat-resistant seal, a gasket, a heating element and the like, and a method for producing the same.

  Film-like graphite is important as an industrial material because it has excellent heat resistance, chemical resistance, high thermal conductivity, and high electrical conductivity, and is widely used as a heat dissipation material, heat-resistant sealing material, gasket, heating element, etc. Yes.

  A representative example of a method for producing artificial film-like graphite is a method called “expanded graphite method”. In this method, artificial graphite is produced by immersing natural graphite in a mixed solution of concentrated sulfuric acid and concentrated nitric acid and then rapidly heating it. The artificial graphite is processed into a film by a high-pressure press after the acid is removed by washing. However, the film-like graphite produced in this way has low strength, other physical characteristic values are not sufficient, and there are also problems such as the influence of residual acid.

  In order to solve such problems, a method of directly heat-treating a special polymer film to graphitize has been developed (hereinafter referred to as “polymer graphitization method”). As the polymer film used for this purpose, for example, a film containing polyoxadiazole, polyimide, polyphenylene vinylene, polybenzimidazole, polybenzoxazole, polythiazole, or polyamide is used. The polymer graphitization method is a much simpler method than the conventional expanded graphite method, and is essentially a method in which impurities such as acids are not mixed. Furthermore, it has excellent heat close to that of single crystal graphite. There is a feature that conductivity and electrical conductivity are obtained (see Patent Documents 1 to 3).

  However, the polymer graphitization method has two problems. The first problem is that it is difficult to obtain a thick film-like graphite in the polymer graphitization method as compared with the expanded graphite method. Various methods have been tried to improve such problems, but at present, conversion to high-quality graphite is possible only when the thickness of the starting material film is up to about 50 μm.

  The second problem is that a long-time heat treatment is required at a very high temperature for graphitization. In general, heat treatment for 30 minutes or more in a temperature region of 2800 ° C. or higher is necessary for conversion to high-quality graphite.

JP-A-60-181129 JP-A-7-109171 JP 61-275116 A

  In view of the problems in the conventional polymer graphitization method as described above, it is an object of the present invention to produce and provide a film-like graphite having a thick and excellent physical property by a short-time heat treatment at a relatively low temperature. It is said.

  In order to solve the above-mentioned problems, the present inventors have focused on polyimide, which is a representative polymer that can be graphitized, and attempted to graphitize various polyimide films. As a result, it was found that conversion to high-quality graphite was possible by controlling the molecular structure and molecular orientation of polyimide. More specifically, it has been found that the birefringence or linear expansion coefficient, which is a physical property of a polyimide film, can be the most direct indicator of whether it can be converted to good quality graphite. The linear expansion coefficient here is a linear expansion coefficient in a direction parallel to the film surface.

  That is, according to the present invention, the method for producing film-like graphite includes the steps of producing a polyimide film having a birefringence of 0.12 or more, and heat-treating the polyimide film at a temperature of 2400 ° C. or more. It is a feature.

In addition, the manufacturing method of a film-form graphite is a step which produces the polyimide film whose average linear expansion coefficient of the film surface direction in the range of 100-200 degreeC is less than 2.5 * 10 < ^-5 > / ^degreeC , and the polyimide film, A step of heat treatment at a temperature of 2400 ° C. or higher may be included.

Moreover, the manufacturing method of a film-form graphite is a polyimide film whose average linear expansion coefficient of the film surface direction in the range of 100-200 degreeC is less than 2.5 * 10 < ^-5 > / degreeC, and birefringence is 0.12 or more. And a step of heat-treating the polyimide film at a temperature of 2400 ° C. or higher.

  In the method for producing film-like graphite, a polyimide film can be produced using an acid dianhydride represented by the following chemical formula 1 as a raw material:

ここで、Ｒ₁は下記化学式２に含まれる２価の有機基の群から選択されるいずれかであり：

Here, R ₁ is any selected from the group of divalent organic groups included in Chemical Formula 2 below:

ここで、Ｒ₂、Ｒ₃、Ｒ₄、およびＲ₅の各々は−ＣＨ₃、−Ｃｌ、−Ｂｒ、−Ｆ、および−ＯＣＨ₃の群から選択されたいずれかである。

Here, each of R ₂ , R ₃ , R ₄ , and R ₅ is any selected from the group of —CH ₃ , —Cl, —Br, —F, and —OCH ₃ .

  In the method for producing film-like graphite, the polyimide film has the following chemical formula 3:

で表される酸二無水物を原料に用いて作製されることが好ましい。

It is preferable to produce using the acid dianhydride represented by these as a raw material.

  In the method for producing film-like graphite, it is also preferable that the polyimide film is produced using pyromellitic dianhydride or p-phenylenediamine as a raw material. In the method for producing film-like graphite, the polyimide film can be produced by allowing a polyamic acid as a precursor to act on a dehydrating agent and an imidization accelerator. The polyimide film uses a first type diamine and an acid dianhydride to synthesize a prepolymer having the acid dianhydride at its terminal, and reacts the prepolymer with a second type diamine to synthesize a polyamic acid. It is preferable that it is produced by imidizing polyamic acid.

The artificial film-like graphite according to the present invention may have a thickness of 30 μm or more and a thermal diffusivity of 8.5 × 10 ⁻⁴ m ² / s or more. The artificial film-like graphite may preferably have a thickness of 3 μm or more and a thermal diffusivity of 10 × 10 ⁻⁴ m ² / s or more.

The artificial film-like graphite may have a thickness of 30 μm or more and an electric conductivity of 8.5 × 10 ⁴ S · cm or more. In the film-like graphite, the electrical resistance ratio at 77K with respect to room temperature can be 1.5 or less, and the electrical resistance ratio at 4K with respect to room temperature can be 1.4 or less.

The artificial film-like graphite can have a thickness of 30 μm or more and a density of 2.15 g / mm ³ or more. In filmy graphite artificial having 30μm or more thick, the peak height of the wave number 1310cm ^-1 against wave number 1580 cm ^-1 in the Raman scattered light when irradiated with a light beam of 10μm diameter at the center of the thickness direction of the cross section The ratio can be 0.35 or less.

FIG. 1 is a plan view illustrating the cutting of a sample for measuring birefringence of a polyimide film. FIG. 2 is a perspective view showing a sample for measuring birefringence cut out in FIG. FIG. 3 is a bright-field image observed by a transmission electron microscope (TEM) in the vicinity of the surface layer of the film-like graphite according to one embodiment of the present invention. FIG. 4 is a lattice image obtained by TEM observation in the vicinity of the surface layer of the film-like graphite according to one embodiment of the present invention. FIG. 5 is a bright-field image obtained by TEM observation near the center of the thickness of the film-like graphite according to one embodiment of the present invention. FIG. 6 is a lattice image obtained by TEM observation near the center of the thickness of the film-like graphite according to one embodiment of the present invention. FIG. 7 is a bright field image obtained by TEM observation in the vicinity of the surface layer of the film-like graphite according to the comparative example. FIG. 8 is a lattice image obtained by TEM observation near the surface layer of the film-like graphite according to the comparative example. FIG. 9 is a bright field image obtained by TEM observation near the center of the thickness of the film-like graphite according to the comparative example. FIG. 10 is a lattice image obtained by TEM observation near the center of the thickness of the film-like graphite according to the comparative example.

  In the polyimide film used in the present invention, the birefringence Δn related to the in-plane orientation of molecules is 0.12 or more, preferably 0.14 or more, most preferably 0.16 or more in any direction in the film plane. It is. If the birefringence of the film is less than 0.12, it indicates that the plane orientation of the molecules of the film is poor, heating to a higher temperature is required for graphitization, and the necessary heat treatment time is also increased. And the electrical conductivity, thermal conductivity, and mechanical strength of the obtained film-like graphite tend to be inferior.

  On the other hand, if the birefringence is 0.12 or more, particularly 0.14 or more, the maximum temperature can be lowered and the heat treatment time can be shortened. Further, since the crystal orientation of the obtained film-like graphite is improved, its electric conductivity, thermal conductivity, and mechanical strength are remarkably improved. The reason for this is not clear, but it is necessary to rearrange the molecules for graphitization, and polyimide with excellent molecular orientation requires minimal molecular rearrangement, enabling graphitization at relatively low temperatures. Presumed to be.

  Birefringence here means the difference between the refractive index in an arbitrary direction in the film plane and the refractive index in the thickness direction, and the birefringence Δnx in the X direction in the film plane is given by the following equation.

Birefringence Δnx = (refractive index Nx in in-plane X direction) − (refractive index Nz in thickness direction)
1 and 2, a specific method for measuring birefringence is illustrated. In the plan view of FIG. 1, a thin wedge-shaped sheet 2 is cut out from the film 1 as a measurement sample. The wedge-shaped sheet 2 has an elongated trapezoidal shape having one oblique side, and its base angle is a right angle. At this time, the base of the trapezoid is cut out in a direction parallel to the X direction. FIG. 2 is a perspective view of the measurement sample 2 cut out in this way. When sodium light 4 is irradiated at right angles to the cut-out cross section corresponding to the bottom side of the trapezoidal sample 2 and observed with a polarizing microscope from the cut-out cross section side corresponding to the oblique side of the trapezoidal sample 2, the interference fringes 5 are observed. If the number of the interference fringes is n, the birefringence Δnx in the X direction in the film plane is
Δnx = n × λ / d
It is represented by Here, λ is the wavelength 589 nm of the sodium D line, and d is the width 3 of the sample corresponding to the trapezoidal height of the sample 2.

  The above-mentioned “arbitrary direction X in the film plane” means, for example, the X direction is the 0 ° direction, 45 ° direction, 90 ° direction, 135 ° direction in the plane with reference to the direction of material flow during film formation. It means in any direction.

Moreover, the polyimide film used as the raw material for the film-like graphite used in the present invention has an average coefficient of linear expansion of less than 2.5 × 10 ⁻⁵ / ° C. in the range of 100 to 200 ° C. By using such a polyimide film as a raw material, conversion to graphite starts at 2400 ° C., and conversion to sufficiently high quality graphite can occur at 2700 ° C. Further, it is less than 2.5 × 10 ⁻⁵ / ° C. as compared with the case where a polyimide film having a linear expansion coefficient of 2.5 × 10 ⁻⁵ / ° C. or higher, which is conventionally known as a raw material for film-like graphite, is used. A polyimide film having a linear expansion coefficient of can be converted to graphite at a lower temperature even with the same thickness. That is, graphitization can easily proceed even if a thicker film than the conventional one is used as a raw material. The linear expansion coefficient is more preferably 2.0 × 10 ⁻⁵ / ° C. or less.

If the linear expansion coefficient of the film is larger than 2.5 × 10 ⁻⁵ / ° C., the change during the heat treatment becomes large, so that the graphitization is disturbed and brittle, and the electric conductivity, thermal conductivity, and The mechanical strength tends to be inferior. On the other hand, if the linear expansion coefficient is less than 2.5 × 10 ⁻⁵ / ° C., the elongation during heat treatment is small, and graphitization proceeds smoothly and is not brittle, and film-like graphite excellent in various properties can be obtained. it can.

  The linear expansion coefficient of the film was determined by using a TMA (thermomechanical analyzer) to first heat the sample to 350 ° C. at a temperature rising rate of 10 ° C./min, then air-cooled to room temperature, and again 10 ° C. / It is obtained by raising the temperature to 350 ° C. at a temperature rise rate of minutes and measuring the average linear expansion coefficient at 100 ° C. to 200 ° C. at the time of the second temperature rise. Specifically, using a thermomechanical analyzer (TMA: Seiko Electronics SSC / 5200H; TMA120C), a 3 mm wide × 20 mm long film sample is set in a predetermined jig, and a load of 3 g is applied in a tensile mode. Is measured in a nitrogen atmosphere.

Moreover, if the elastic modulus of the polyimide film used in the present invention is 350 kgf / mm ² or more, it is preferable because graphitization can be performed more easily. That is, if the elastic modulus is 350 kgf / mm ² or more, it becomes possible to perform heat treatment while applying tension to the polyimide film, and it is possible to prevent the film from being damaged due to the shrinkage of the film during the heat treatment. Excellent film-like graphite can be obtained.

In addition, the elasticity modulus of a film can be measured based on ASTM-D-882. More preferred elastic modulus of the polyimide film at 400 kgf / mm ² or more, further preferably 500 kgf / mm ² or more. If the elastic modulus of the film is smaller than 350 kgf / mm ² , the film is easily damaged and deformed by shrinkage of the film during heat treatment, and the resulting film-like graphite tends to be inferior in electrical conductivity, thermal conductivity, and mechanical strength.

  The polyimide film used in the present invention can be produced by casting an organic solution of polyamic acid, which is a polyimide precursor, on a support such as an endless belt or a stainless drum, and drying and imidizing it.

  As a method for producing the polyamic acid used in the present invention, a known method can be used. Usually, at least one kind of aromatic dianhydride and at least one kind of diamine are substantially equimolar amounts in an organic solvent. Dissolved in. The obtained organic solution is stirred under controlled temperature conditions until the polymerization of the acid dianhydride and the diamine is completed, whereby a polyamic acid can be produced. Such a polyamic acid solution is usually obtained at a concentration of 5 to 35 wt%, preferably 10 to 30 wt%. When the concentration is in this range, an appropriate molecular weight and solution viscosity can be obtained.

  Any known method can be used as the polymerization method. For example, the following polymerization methods (1) to (5) are preferable.

  (1) A method in which an aromatic diamine is dissolved in an organic polar solvent, and this is reacted with a substantially equimolar aromatic tetracarboxylic dianhydride for polymerization.

  (2) An aromatic tetracarboxylic dianhydride is reacted with a small molar amount of an aromatic diamine compound in an organic polar solvent to obtain a prepolymer having acid anhydride groups at both ends. Then, the method of superposing | polymerizing using an aromatic diamine compound so that it may become substantially equimolar with respect to aromatic tetracarboxylic dianhydride.

  (3) An aromatic tetracarboxylic dianhydride and an excess molar amount of the aromatic diamine compound are reacted in an organic polar solvent to obtain a prepolymer having amino groups at both ends. Subsequently, after adding an aromatic diamine compound to the prepolymer, polymerization was performed using the aromatic tetracarboxylic dianhydride so that the aromatic tetracarboxylic dianhydride and the aromatic diamine compound are substantially equimolar. how to.

  (4) After dissolving and / or dispersing aromatic tetracarboxylic dianhydride in an organic polar solvent, an aromatic diamine compound is used so as to be substantially equimolar with respect to the acid dianhydride. Polymerization method.

  (5) A method of polymerizing by reacting a substantially equimolar mixture of aromatic tetracarboxylic dianhydride and aromatic diamine in an organic polar solvent.

  Among these, as in (2) and (3), a method of polymerizing by sequential control via a prepolymer is preferable. Because, by sequential control, it is easy to obtain a polyimide film having a small birefringence and a small linear expansion coefficient. By heat-treating this polyimide film, a film having excellent electrical conductivity, thermal conductivity, and mechanical strength. This is because it becomes easier to obtain graphite. Further, it is presumed that when the polymerization reaction is regularly controlled, overlapping of aromatic rings increases, and graphitization is likely to proceed even at low temperature heat treatment.

  Acid dianhydrides that can be used for the synthesis of polyimide in the present invention are pyromellitic dianhydride, 2,3,6,7-naphthalenetetracarboxylic dianhydride, 3,3 ′, 4,4′-biphenyl. Tetracarboxylic dianhydride, 1,2,5,6-naphthalene tetracarboxylic dianhydride, 2,2 ′, 3,3′-biphenyltetracarboxylic dianhydride, 3,3 ′, 4,4 ′ -Benzophenonetetracarboxylic dianhydride, 2,2-bis (3,4-dicarboxyphenyl) propane dianhydride, 3,4,9,10-perylenetetracarboxylic dianhydride, bis (3,4- Dicarboxyphenyl) propane dianhydride, 1,1-bis (2,3-dicarboxyphenyl) ethane dianhydride, 1,1-bis (3,4-dicarboxyphenyl) ethane dianhydride, bis (2 , 3-Zical Xylphenyl) methane dianhydride, bis (3,4-dicarboxyphenyl) ethane dianhydride, oxydiphthalic dianhydride, bis (3,4-dicarboxyphenyl) sulfone dianhydride, p-phenylenebis (trimerit Acid monoester acid anhydride), ethylene bis (trimellitic acid monoester acid anhydride), bisphenol A bis (trimellitic acid monoester acid anhydride), and the like, which may be used alone or in any It can be used in a mixture of proportions.

  Examples of the diamine that can be used in the synthesis of the polyimide in the present invention include 4,4′-oxydianiline, p-phenylenediamine, 4,4′-diaminodiphenylpropane, 4,4′-diaminodiphenylmethane, benzidine, 3,3. '-Dichlorobenzidine, 4,4'-diaminodiphenyl sulfide, 3,3'-diaminodiphenyl sulfone, 4,4'-diaminodiphenyl sulfone, 4,4'-diaminodiphenyl ether, 3,3'-diaminodiphenyl ether, 3, 4'-diaminodiphenyl ether, 1,5-diaminonaphthalene, 4,4'-diaminodiphenyldiethylsilane, 4,4'-diaminodiphenylsilane, 4,4'-diaminodiphenylethylphosphine oxide, 4,4'-diaminodiphenyl N-methylamine 4,4'-diaminodiphenyl N-phenylamine, 1,4-diaminobenzene (p-phenylenediamine), 1,3-diaminobenzene, 1,2-diaminobenzene and the like and including them alone Or it can be used in the mixture of arbitrary ratios.

  In particular, from the viewpoint of increasing the elastic modulus and increasing the birefringence by reducing the linear expansion coefficient, in the production of the polyimide film in the present invention, an acid dianhydride represented by the following chemical formula 1 is used as a raw material. preferable.

Here, R ₁ is any one selected from the group of divalent organic groups included in the following chemical formula (2),

ここで、Ｒ₂、Ｒ₃、Ｒ₄、およびＲ₅の各々は−ＣＨ₃、−Ｃｌ、−Ｂｒ、−Ｆ、または−ＣＨ₃Ｏの群から選択されるいずれかであり得る。

Here, each of R ₂ , R ₃ , R ₄ , and R ₅ can be any selected from the group of —CH ₃ , —Cl, —Br, —F, or —CH ₃ O.

  By using the above acid dianhydride, a polyimide film having a relatively low water absorption can be obtained, which is also preferable from the viewpoint of preventing foaming due to moisture in the graphitization process.

In particular, when an organic group containing a benzene nucleus as shown in Chemical Formula 2 is used as R ₁ in the acid dianhydride, the molecular orientation of the resulting polyimide film is increased, the linear expansion coefficient is small, and the elastic modulus Is preferable from the viewpoints of large, high birefringence, and low water absorption.

  In order to further reduce the linear expansion coefficient, increase the elastic modulus, increase the birefringence, and decrease the water absorption, an acid dianhydride represented by the following molecular formula 3 may be used as a raw material for the synthesis of the polyimide in the present invention. .

In particular, a polyimide film obtained by using an acid dianhydride having a structure in which a benzene ring is linearly bonded by two or more ester bonds as a raw material has a very linear conformation as a whole although it contains a bent chain. It has a relatively rigid property. As a result, the linear expansion coefficient of the polyimide film can be reduced by using this raw material, for example, 1.5 × 10 ⁻⁵ / ° C. or less. Further, the elastic modulus can be increased to 500 kgf / mm ² or more, and the water absorption can be decreased to 1.5% or less.

  In order to further reduce the linear expansion coefficient, increase the elastic modulus, and increase the birefringence, the polyimide in the present invention is preferably synthesized using p-phenylenediamine as a raw material.

  In the present invention, the most suitable acid dianhydride used for the synthesis of the polyimide film is pyromellitic dianhydride and / or p-phenylenebis (trimellitic acid monoester dianhydride) represented by (Chemical Formula 3). And the total mole of these alone or the two is 40 mole% or more, further 50 mole% or more, further 70 mole% or more, or even 80 mole% or more based on the total acid dianhydride. Is preferred. If the amount of these acid dianhydrides used is less than 40 mol%, the resulting polyimide film tends to have a large linear expansion coefficient, a small elastic modulus, and a small birefringence.

  In the present invention, the most suitable diamines used for the synthesis of polyimide are 4,4′-oxydianiline and p-phenylenediamine, and the total moles of these alone or the two are 40% by mole based on the total diamines. More preferably, it is 50 mol% or more, more preferably 70 mol% or more, and even more preferably 80 mol% or more. Furthermore, it is preferable that p-phenylenediamine contains 10 mol% or more, further 20 mol% or more, further 30 mol% or more, and further 40 mol% or more. If the content of these diamines is less than the lower limit of these mol% ranges, the resulting polyimide film tends to have a high linear expansion coefficient, a low elastic modulus, and a low birefringence. However, if the total amount of diamine is p-phenylenediamine, it is difficult to obtain a thick polyimide film with less foaming, so 4,4'-oxydianiline is preferably used.

  Preferred solvents for synthesizing the polyamic acid are amide solvents N, N-dimethylformamide, N, N-dimethylacetamide, N-methyl-2-pyrrolidone and the like, N, N-dimethylformamide, N, N-dimethylacetamide can be used particularly preferably.

  Next, a polyimide production method includes a heat curing method in which polyamic acid as a precursor is converted to imide by heating, or a polyhydric acid such as acetic anhydride and other dehydrating agents, picoline, quinoline, isoquinoline, Any of the chemical curing methods in which imide conversion is performed using a tertiary amine such as pyridine as an imidization accelerator may be used. Among them, a higher boiling point such as isoquinoline is preferable. This is because it does not evaporate in the initial stage of film production and tends to exert a catalytic effect until the last stage of drying.

  In particular, the resulting film has a low coefficient of linear expansion, high modulus of elasticity, high birefringence, and can be rapidly graphitized at a relatively low temperature to obtain high-quality graphite. Cure is preferred. Moreover, it is preferable to use a dehydrating agent and an imidization accelerator in combination because the obtained film has a small linear expansion coefficient, a large elastic modulus, and a large birefringence. Furthermore, the chemical cure method is an industrially advantageous method that is excellent in productivity because the imidization reaction proceeds faster and the imidization reaction can be completed in a short time in the heat treatment.

  In the production of a film by a specific chemical cure, first, an imidization accelerator comprising a dehydrating agent and a catalyst of a stoichiometric amount or more is added to a polyamic acid solution, an organic film such as a support plate, PET, a drum, or an endless belt. A film having self-supporting properties is obtained by casting or coating on a support such as a film to evaporate the organic solvent. Next, the self-supporting film is further heated and dried to be imidized to obtain a polyimide film. The temperature during the heating is preferably in the range of 150 ° C to 550 ° C.

  There is no particular limitation on the rate of temperature increase at the time of heating, but it is preferable to gradually heat in a continuous or stepwise manner so that the maximum temperature falls within the predetermined temperature range. Although the heating time varies depending on the film thickness and the maximum temperature, it is generally preferably in the range of 10 seconds to 10 minutes after reaching the maximum temperature. Furthermore, if the process of fixing or stretching the film to prevent shrinkage is included in the polyimide film manufacturing process, the resulting film has a low coefficient of linear expansion, a high elastic modulus, and a high birefringence. Since it tends to be easy, it is preferable.

  In the process of graphitization of a polyimide film, in the present invention, the starting polyimide film is carbonized by preheating treatment under reduced pressure or nitrogen gas. This preheating is usually performed at a temperature of about 1000 ° C., and for example, when the temperature is raised at a rate of 10 ° C./min, it is desirable to hold for about 30 minutes in the temperature range of 1000 ° C. In the temperature raising stage, it is preferable to apply pressure in a direction perpendicular to the film surface to such an extent that the film is not damaged so that the molecular orientation of the starting polymer film is not lost.

  Next, the carbonized film is set in an ultrahigh temperature furnace and graphitized. Graphitization is carried out in an inert gas. Argon is suitable as the inert gas, and it is more preferable to add a small amount of helium to argon. The heat treatment temperature is required to be at least 2400 ° C., and finally, heat treatment is preferably performed at a temperature of 2700 ° C. or more, and heat treatment is more preferably performed at 2800 ° C. or more.

  The higher the heat treatment temperature is, the higher the quality can be converted to graphite, but from the viewpoint of economy, it is preferable that the conversion to high quality graphite is possible at the lowest possible temperature. In order to obtain an ultra-high temperature of 2500 ° C. or higher, heating is usually performed by passing a current directly through the graphite heater and utilizing its juule heat. The consumption of the graphite heater proceeds at 2700 ° C. or more, and the consumption rate becomes about 10 times at 2800 ° C., and further about 10 times at 2900 ° C. Therefore, by improving the raw material polymer film, lowering the temperature at which conversion to high-quality graphite can be performed, for example, from 2800 ° C. to 2700 ° C. has a great economic effect. In a generally available industrial furnace, the maximum heat-treatable temperature is 3000 ° C.

  In the graphitization treatment, the carbonized film produced by the preliminary heat treatment is converted into a graphite structure, and in that case, the carbon-carbon bond must be cleaved and recombined. In order for graphitization to occur at the lowest possible temperature, its cleavage and recombination must occur with minimal energy. The molecular orientation of the starting polyimide film affects the arrangement of carbon atoms in the carbonized film, which can have the effect of reducing the energy of carbon-carbon bond cleavage and recombination during graphitization. Therefore, graphitization at a relatively low temperature becomes possible by designing a molecule so that a high degree of molecular orientation is likely to occur. The effect of this molecular orientation becomes more prominent by adopting a two-dimensional molecular orientation parallel to the film surface.

  The second feature of the graphitization reaction is that if the carbonized film is thick, the graphitization hardly proceeds at low temperatures. Therefore, when graphitizing a thick carbonized film, a situation may occur in which a graphite structure is formed in the surface layer but not yet formed into a graphite structure. The molecular orientation of the carbonized film promotes graphitization inside the film and consequently allows conversion to good quality graphite at lower temperatures.

  The fact that graphitization proceeds almost simultaneously in the surface layer and inside of the carbonized film also helps to avoid the situation where the graphite structure formed in the surface layer is destroyed due to the gas generated from the inside, Enables graphitization of thicker films. The polyimide film produced in the present invention is considered to have an optimal molecular orientation for producing such an effect.

  As described above, if the polyimide film produced in the present invention is used, it becomes possible to graphitize a film thicker than a conventional graphitizable polyimide film. Specifically, even a film having a thickness of 200 μm can be converted into high-quality film-like graphite by selecting an appropriate heat treatment.

  In the following, various embodiments of the present invention will be described along with some comparative examples.

Example 1
In a DMF (dimethylformamide) solution in which 3 equivalents of 4,4′-oxydianiline and 1 equivalent of p-phenylenediamine are dissolved, 4 equivalents of pyromellitic dianhydride are dissolved to obtain polyamic acid in 18. A solution containing 5 wt% was obtained.

  While this solution was cooled, an imidation catalyst containing 1 equivalent of acetic anhydride, 1 equivalent of isoquinoline, and DMF was added to the carboxylic acid group contained in the polyamic acid to degas. Next, this mixed solution was applied onto an aluminum foil so as to have a predetermined thickness after drying. The mixed solution layer on the aluminum foil was dried using a hot air oven and a far infrared heater.

  The drying conditions when the finished thickness is 75 μm are as follows. The mixed solution layer on the aluminum foil was dried in a hot air oven at 120 ° C. for 240 seconds to form a self-supporting gel film. The gel film was peeled off from the aluminum foil and fixed to the frame. The gel film is heated stepwise in a hot air oven for 30 seconds at 120 ° C, 40 seconds at 275 ° C, 43 seconds at 400 ° C, 50 seconds at 450 ° C, and 23 seconds at 460 ° C in a far infrared heater. And dried. For other thicknesses, the firing time was adjusted in proportion to the thickness. For example, in the case of a film having a thickness of 25 μm, the firing time was set to be 1/3 shorter than that in the case of 75 μm.

Five types of polyimide films having a thickness of 25 μm, 50 μm, 75 μm, 100 μm, and 200 μm (sample A: elastic modulus 400 kgf / mm ² , water absorption> 2.0%) were manufactured.

Sample A was sandwiched between graphite plates, and pre-treatment was performed using an ultra-high temperature furnace having a graphite heater and the temperature was raised to 1000 ° C. at a rate of 16.7 ° C./min under reduced pressure. Subsequently, the temperature was raised to 2700 ° C. at a rate of 7 ° C./min in a pressurized argon atmosphere of 0.8 kgf / cm ² using an ultrahigh temperature furnace. Further, in a pressurized argon atmosphere of 0.8 kgf / cm ^2, the temperature was raised to a maximum temperature of 2800 ° C. at a rate of 2 ° C./min, and the maximum temperature was maintained for 1 hour. Thereafter, it was cooled to obtain a film-like graphaite.

  The progress of graphitization was determined by measuring the electric conductivity and thermal diffusivity in the film surface direction. That is, the higher the electric conductivity and the thermal diffusivity, the more remarkable the graphitization. The results are shown in Table 1. In the case of the polyimide of Example 1 (sample A), conversion to high-quality graphite has already occurred by heat treatment at 2700 ° C., and excellent characteristics are obtained with respect to both electrical conductivity and thermal conductivity. . If the polyimide of Example 1 is used, compared with the conventional Kapton (trademark) type polyimide shown in Comparative Example 1 described below, it is possible to graphitize even with a thick polyimide film. It has been found that conversion to high-quality graphite is possible even at a temperature of 2700 ° C., which is lower than the graphitization temperature of 2800 ° C. by 100 ° C.

  The electrical conductivity was measured by the 4-terminal method. Specifically, first, a film-like graphite sample having a size of about 3 mm × 6 mm was prepared. After confirming that the sample was not torn or wrinkled with an optical microscope, a pair of outer electrodes were attached with silver paste to both ends of the sample, and a pair of inner electrodes were attached with silver paste between the outer electrodes. A constant current source ("Programmable Current Source 220" available from Keithley Co., Ltd.) is used to apply a constant current of 1 mA between the outer electrodes, and the voltage between the inner electrodes is measured by a voltmeter (from Keithley Co., Ltd.). It was measured with an available “Nanovoltmeter 181”). The electrical conductivity was calculated using the formula of (applied current / measured voltage) × (inner electrode distance / sample cross-sectional area).

  The thermal diffusivity was measured at 10 Hz in an atmosphere of 20 ° C. using a thermal diffusivity measuring apparatus (“LaserPit” available from ULVAC Riko Co., Ltd.) by an optical alternating current method.

(Comparative Example 1)
One equivalent of pyromellitic dianhydride was dissolved in a DMF solution in which one equivalent of 4,4′-oxydianiline was dissolved to obtain a solution containing 18.5 wt% of polyamic acid.

  While this solution was cooled, an imidation catalyst containing 1 equivalent of acetic anhydride, 1 equivalent of isoquinoline, and DMF was added to the carboxylic acid group contained in the polyamic acid to degas. Next, this mixed solution was applied onto an aluminum foil so as to have a predetermined thickness after drying. The mixed solution layer on the aluminum foil was dried using a hot air oven and a far infrared heater.

  The drying conditions when the finished thickness is 75 μm are as follows. The mixed solution layer on the aluminum foil was dried in a hot air oven at 120 ° C. for 240 seconds to form a self-supporting gel film. The gel film was peeled off from the aluminum foil and fixed to the frame. Furthermore, the gel film is heated stepwise in a hot air oven at 120 ° C. for 30 seconds, 275 ° C. for 40 seconds, 400 ° C. for 43 seconds, 450 ° C. for 50 seconds, and a far infrared heater at 460 ° C. for 23 seconds. Has been dried. For other thicknesses, the firing time was adjusted in proportion to the thickness. For example, in the case of a film having a thickness of 25 μm, the firing time was set to be 1/3 shorter than that in the case of 75 μm.

Five conventional representative Kapton (trademark) type polyimide films (elastic modulus 300 kgf / mm ² , water absorption> 2.0%) having thicknesses of 25 μm, 50 μm, 75 μm, 100 μm, and 200 μm were produced. Using these films, graphitization was performed in the same manner as in Example 1 also in this comparative example.

  Table 2 shows the characteristics of the film-like graphite obtained in Comparative Example 1. In Table 2, when a polyimide film having a thickness of 75 μm or more was used, only graphite having inferior quality in electrical conductivity and thermal diffusivity was obtained, but only a polyimide film having a thickness of 25 μm and 50 μm had a high degree. Graphitization was obtained. Further, as can be seen from Table 2, the characteristics of the graphitized film obtained by the heat treatment at 2700 ° C. in this Comparative Example 1 are considerably inferior compared with the case of using the polyimide of Example 1 (Sample A). Met.

  From the comparison between Comparative Example 1 and Example 1 as described above, the superiority of the polyimide of the present invention in the graphitization reaction was clarified.

(Example 2)
In a DMF (dimethylformamide) solution in which 2 equivalents of 4,4′-oxydianiline and 1 equivalent of p-phenylenediamine are dissolved, 3 equivalents of pyromellitic dianhydride is dissolved to obtain 15 wt% polyamic acid. A solution containing was obtained.

  While this solution was cooled, an imidation catalyst containing 1 equivalent of acetic anhydride, 1 equivalent of isoquinoline, and DMF was added to the carboxylic acid group contained in the polyamic acid to degas. Next, this mixed solution was applied onto an aluminum foil so as to have a predetermined thickness after drying. The mixed solution layer on the aluminum foil was dried using a hot air oven and a far infrared heater.

  The drying conditions when the finished thickness is 75 μm are as follows. The mixed solution layer on the aluminum foil was dried in a hot air oven at 120 ° C. for 240 seconds to form a self-supporting gel film. The gel film was peeled off from the aluminum foil and fixed to the frame. Furthermore, the gel film is heated stepwise in a hot air oven at 120 ° C. for 30 seconds, 275 ° C. for 40 seconds, 400 ° C. for 43 seconds, 450 ° C. for 50 seconds, and a far infrared heater at 460 ° C. for 23 seconds. Has been dried. For other thicknesses, the firing time was adjusted in proportion to the thickness. For example, in the case of a film having a thickness of 25 μm, the firing time was set to be 1/3 shorter than that in the case of 75 μm.

  In addition, compared with Example 1, in this Example 2, the ratio of p-phenylenediamine which is a rigid component is large, and the molecular orientation of the finished polyimide film is advanced. Therefore, in the case of a thick film, the solvent or catalyst is caught in the resin, and foaming due to evaporation of the solvent or imidization catalyst of the polyimide film is likely to occur, and the firing time at low temperature is sufficiently long to prevent foaming. It was necessary to set.

  Four types of polyimide films (sample B: elastic modulus 450 kg / mm 2, water absorption> 2.0%) with thicknesses of 25 μm, 50 μm, 75 μm, and 100 μm were produced. Using these films, graphitization was also performed in Example 2 in the same manner as in Example 1.

  The characteristics of the film-like graphite obtained in Example 2 are shown in Table 3. As can be seen from a comparison between Table 3 and Table 1, the properties of the film-like graphite obtained in Example 2 were slightly superior to Example 1.

(Example 3)
1 equivalent of pyromellitic dianhydride, p-phenylenebis (trimellitic acid monoester anhydride) in a DMF solution in which 1 equivalent of 4,4′-oxydianiline and 1 equivalent of p-phenylenediamine are dissolved Was dissolved to obtain a solution containing 15% by weight of polyamic acid.

  While this solution was cooled, an imidation catalyst containing 1 equivalent of acetic anhydride, 1 equivalent of isoquinoline, and DMF was added to the carboxylic acid group contained in the polyamic acid to degas. Next, this mixed solution was applied onto an aluminum foil so as to have a predetermined thickness after drying. The mixed solution layer on the aluminum foil was dried using a hot air oven and a far infrared heater.

  The drying conditions when the finished thickness is 75 μm are as follows. The mixed solution layer on the aluminum foil was dried in a hot air oven at 120 ° C. for 240 seconds to form a self-supporting gel film. The gel film was peeled off from the aluminum foil and fixed to the frame. Furthermore, the gel film is heated stepwise in a hot air oven at 120 ° C. for 30 seconds, 275 ° C. for 40 seconds, 400 ° C. for 43 seconds, 450 ° C. for 50 seconds, and a far infrared heater at 460 ° C. for 23 seconds. Has been dried. For other thicknesses, the firing time was adjusted in proportion to the thickness. For example, in the case of a film having a thickness of 25 μm, the firing time was set to be 1/3 shorter than that in the case of 75 μm.

  Moreover, compared with Example 1, in this Example 3, the molecular orientation of the completed polyimide film is advanced. Therefore, in the case of a thick film, the solvent and catalyst are caught in the resin, and foaming due to evaporation of the solvent and imidization catalyst of the polyimide film is likely to occur, and the firing time at low temperature is set sufficiently long to prevent foaming. There was a need to do.

Four types of polyimide films (sample C: elastic modulus 500 kgf / mm ² , water absorption> 1.5%) having thicknesses of 25 μm, 50 μm, 75 μm, and 100 μm were manufactured. Using these films, graphitization was also performed in Example 3 in the same manner as in Example 1.

  Table 4 shows the properties of the film-like graphite obtained in Example 3. As can be seen from the comparison between Table 4 and Table 1, the characteristics of the film-like graphite obtained in Example 3 were almost the same as those in Example 1.

Example 4
In a DMF (dimethylformamide) solution in which 3 equivalents of 4,4′-oxydianiline and 1 equivalent of p-phenylenediamine are dissolved, 4 equivalents of pyromellitic dianhydride are dissolved to obtain polyamic acid in 18. A solution containing 5 wt% was obtained.

  While this solution was cooled, an imidation catalyst containing 1 equivalent of isoquinoline and DMF was added to the carboxylic acid group contained in the polyamic acid to degas. Next, this mixed solution was applied onto an aluminum foil so as to have a predetermined thickness after drying. The mixed solution layer on the aluminum foil was dried using a hot air oven.

  The drying conditions when the finished thickness is 75 μm are as follows. The mixed solution layer on the aluminum foil was dried in a hot air oven at 120 ° C. for 240 seconds to form a self-supporting gel film. The gel film was peeled off from the aluminum foil and fixed to the frame. Further, the gel film was dried by heating stepwise in a hot air oven at 120 ° C. for 30 minutes, 275 ° C. for 30 minutes, 400 ° C. for 30 minutes, and 450 ° C. for 30 minutes. For other thicknesses, the firing time was adjusted in proportion to the thickness. For example, in the case of a film having a thickness of 25 μm, the firing time was set to be 1/3 shorter than that in the case of 75 μm. Moreover, when the film was thick, in order to prevent foaming due to evaporation of the solvent of the polyimide film or imidization catalyst, the firing time at low temperature was set longer. In particular, when acetic anhydride is not added as in Example 4, the progress of the reaction is slow and the polarity does not change, so that the leaching of the solvent and catalyst is slow, and foaming is likely to occur during polyimide film production. Therefore, when producing a thick polyimide film, it is necessary to pay sufficient attention to the drying conditions.

Four types of polyimide films (sample D: elastic modulus 380 kgf / mm ² , water absorption> 2.2%) having thicknesses of 25 μm, 50 μm, 75 μm, and 100 μm were manufactured. Using these films, graphitization was also performed in Example 4 in the same manner as in Example 1.

  The properties of the film-like graphite obtained in Example 4 are shown in Table 5. As can be seen from the comparison between Table 5 and Tables 1 and 2, the characteristics of the film-like graphite obtained in Example 4 were slightly inferior to those of Example 1, but were superior to those of Comparative Example.

(Example 5)
In Example 5, polyimide films of various thicknesses manufactured by Kaneka Chemical Industry Co., Ltd. and sold under the trade name Apical NPI were graphitized in the same manner as in Example 1.

  The manufacturing method of apical NPI is as follows. 4 equivalents of pyromellitic dianhydride was dissolved in a DMF solution in which 3 equivalents of 4,4'-oxydianiline was dissolved to synthesize a prepolymer having acid anhydrides at both ends. Furthermore, a solution containing 18.5 wt% of polyamic acid was obtained by dissolving 1 equivalent of p-phenylenediamine in the solution containing the prepolymer.

  While this solution was cooled, an imidation catalyst containing 1 equivalent of isoquinoline and DMF was added to the carboxylic acid group contained in the polyamic acid to degas. Next, this mixed solution was applied onto an aluminum foil so as to have a predetermined thickness after drying. The mixed solution layer on the metal belt was dried using a hot air oven and a far infrared heater.

  The drying conditions when the finished thickness is 75 μm are as follows. The mixed solution layer on the metal belt was dried in a hot air oven at 120 ° C. for 240 seconds to form a self-supporting gel film. The gel film was peeled off from the metal belt and the end was fixed. Furthermore, the gel film is heated stepwise in a hot air oven at 120 ° C. for 30 seconds, 275 ° C. for 40 seconds, 400 ° C. for 43 seconds, 450 ° C. for 50 seconds, and a far infrared heater at 460 ° C. for 23 seconds. Has been dried. For other thicknesses, the firing time was adjusted in proportion to the thickness. For example, in the case of a film having a thickness of 25 μm, the firing time was set to be 1/3 shorter than that in the case of 75 μm.

Five types of sequentially controlled polyimide films with thicknesses of 12.5 μm, 25 μm, 50 μm, 75 μm, and 125 μm (Sample E: elastic modulus 380 kgf / mm ² , water absorption 2.2%, birefringence 0.14, linear expansion) A factor of 1.6 × 10 ⁻⁵ / ° C.) was produced. Using these films, graphitization was also performed in Example 5 in the same manner as in Example 1. However, in Example 5, the heat treatment temperature was 2800 ° C. or 3000 ° C.

  Various physical properties of the film-like graphite obtained in Example 5 are shown in Table 6. As can be seen from the comparison between Table 6 and Tables 1 to 5, the properties of the film-like graphite of Example 5 are superior to those of Examples 1 to 4 as well as Comparative Example 1. As a matter of course, density and electric conductivity tend to increase as graphitization proceeds. In Table 6, ρ (77K) / ρ (rt) and ρ (4K) / ρ (rt) represent the electrical resistance ratios at 77K and 4K, respectively, with respect to room temperature. It tends to decrease.

Furthermore, thermal conductivity and thermal diffusivity tend to increase as graphitization proceeds, and these characteristics are directly important when film-like graphite is used as a heat dissipation film. Here, the thermal conductivity (W / (m · K)) is the thermal diffusivity (m ² / s), density (kg / m ³ ) and specific heat (theoretical value: 0.709 kJ / (kg · K)). Calculated by multiplying. The density was calculated by dividing weight by volume.

The Raman spectral intensity ratio in Table 6 is represented by the ratio of the spectral peak at wave number 1310 cm ^-1 corresponding to diamond bond to the spectral peak at wave number 1580 cm ^-1 corresponding to graphite bond. Of course, the smaller the spectral peak ratio, the more graphitized. In this Raman measurement, a light beam having a diameter of 10 μm was applied to the center of the cross section in the thickness direction of the film-like graphite.

  In FIG. 3, the bright field image by the transmission electron microscope (TEM) observation of the surface layer vicinity of the film-like graphite obtained by heat processing at 3000 degreeC from the 125-micrometer-thick polyimide film in Example 5 is shown. In the TEM observation, a film-like graphite was embedded in a protective resin, and a sample for transmission observation of a cross section in the thickness direction of the graphite layer was produced. In FIG. 3, the arrow indicates the boundary between the protective resin and the graphite layer.

  As can be seen from the layered contrast of the TEM photograph, the graphite layer has a single crystal structure in which the crystallographic (0001) plane (generally also referred to as “C plane”) is parallel to the surface. In FIG. 3, delamination along the C-plane is observed, but these delaminations were introduced by an unexpected external force during the preparation and handling of the microscope sample. The fact that such delamination tends to occur means that graphite crystallization has advanced to a high degree and is easily broken along the C plane.

  4 shows a crystal lattice image obtained by TEM observation in the vicinity of the surface layer corresponding to FIG. In FIG. 4, the line-shaped lattice image corresponding to the C-plane of graphite appears with clear contrast, and it can be confirmed that these line-shaped lattices continue long in parallel to the surface.

  FIG. 5 is similar to FIG. 3, but shows a bright field image obtained by TEM observation near the center in the thickness direction of the film-like graphite. From this TEM photograph, it can be seen that a graphite single crystal-like structure similar to that of the surface layer is formed in the vicinity of the center of the thickness. Also in FIG. 5, delamination along the C-plane is observed, similar to FIG. FIG. 6 shows a crystal lattice image obtained by TEM observation near the center in the thickness direction corresponding to FIG. In FIG. 6 as well, the linearity of the line-like lattice image corresponding to the C-plane of graphite is slightly inferior to that in FIG. 4, but it can be confirmed that these lines continue for a long time.

(Comparative Example 2)
In Comparative Example 2, polyimide films of various thicknesses manufactured by DuPont and sold under the trade name Kapton H were graphitized in the same manner as in Example 5.

  The production method of Kapton film is not clear, but a polyamic acid solution obtained by dissolving 1 equivalent of 4,4′-oxydianiline in DMAc (dimethylacetamide) and further dissolving 1 equivalent of pyromellitic dianhydride It is presumed that it is produced by adding an imidization catalyst consisting of acetic anhydride, betapicoline, and DMAc.

The elastic modulus of Kapton H is 330 kgf / mm ² , the water absorption is 2.9%, the birefringence is 0.11, and the linear expansion coefficient is 2.7 × 10 ⁻⁵ / ° C.

  Various physical properties of the film-like graphite obtained in Comparative Example 2 are shown in Table 7. As can be seen from the comparison between Table 7 and Table 6, it is clear that the characteristics of the film-like graphite of Example 5 are significantly superior to those of the film-like graphite obtained from Comparative Example 2 Kapton film. is there.

  FIG. 7 shows a bright-field image obtained by TEM observation in the vicinity of the surface layer of film-like graphite obtained from a Kapton (trademark) film having a thickness of 125 μm in Comparative Example 2 by a heat treatment at 3000 ° C. Also in FIG. 7, as can be seen from the layered contrast, a single crystal structure in which the C-plane of graphite is parallel to the surface is formed in the vicinity of the surface layer. In FIG. 8, the crystal lattice image by the TEM observation of the surface layer vicinity corresponding to FIG. 7 is shown. In FIG. 8 as well, the linearity of the line-like lattice image corresponding to the C-plane of graphite is slightly inferior to that of FIG. 4, but it can be confirmed that the line continues for a long time.

  FIG. 9 shows a bright field image obtained by TEM observation near the center in the thickness direction of the film-like graphite. In this TEM photograph, it is clear that the layered structure is not formed near the center of the thickness, unlike the surface layer, and the graphitization is insufficient. In FIG. 10, a crystal lattice image obtained by TEM observation near the center in the thickness direction is shown corresponding to FIG. In FIG. 10, it can be confirmed that the line-shaped lattice image corresponding to the C-plane of graphite is wavy or broken. This indicates that even when graphitization has partially progressed, the generated graphite is in a microcrystalline state or the C-plane orientation is not aligned. That is, in the film-like graphite obtained from the Kapton (trademark) film, it can be seen that even when heat-treated at a high temperature of 3000 ° C., graphitization does not proceed sufficiently at the central portion of the thickness.

  According to the present invention, it is possible to produce a thick film-like graphite as compared with the conventional polymer graphitization method, and when graphitizing a polymer film having the same thickness, the graphite can be obtained at a lower temperature in a short time. Is possible.

Claims (13)

An artificial film-like graphite obtained by heat-treating a polyimide film having a birefringence of 0.12 or more at a temperature of 2800 ° C. or less, and having a thermal conductivity of 1305 W / m · K or more Graphite. An artificial film-like graphite obtained by heat-treating a polyimide film having a birefringence of 0.12 or more at a temperature of 3000 ° C. or less, and having a thermal conductivity of 1725 W / m · K or more Graphite. An artificial film-like graphite obtained by heat-treating a polyimide film having a thickness of 12.5 μm or more and birefringence of 0.12 or more, and having a thermal conductivity of 1767 W / m · K or more. Film-like graphite. An artificial film-like graphite obtained by heat-treating a polyimide film having a birefringence of 0.12 or more, having a thickness of 4.9 μm or more, and a thermal conductivity of 1767 W / m · K or more. Film-like graphite characterized by An artificial film-like graphite obtained by heat-treating a polyimide film having a thickness of 25 μm or more and a birefringence of 0.12 or more , and having a thermal conductivity of 1760 W / m · K or more. Graphite. An artificial film-like graphite obtained by heat-treating a polyimide film having a birefringence of 0.12 or more, a thickness of 10.6 μm or more, and a thermal conductivity of 1760 W / m · K or more. Film-like graphite characterized by An artificial film-like graphite obtained by heat-treating a polyimide film having a thickness of 50 μm or more and a birefringence of 0.12 or more, and having a thermal conductivity of 1725 W / m · K or more. Graphite. An artificial film-like graphite obtained by heat-treating a polyimide film having a birefringence of 0.12 or more, having a thickness of 20 μm or more and a thermal conductivity of 1725 W / m · K or more. Film-like graphite. An artificial film-like graphite obtained by heat-treating a polyimide film having a thickness of 75 μm or more and a birefringence of 0.12 or more, and having a thermal conductivity of 1500 W / m · K or more. Graphite. An artificial film-like graphite obtained by heat-treating a polyimide film having a birefringence of 0.12 or more, having a thickness of 33 μm or more and a thermal conductivity of 1500 W / m · K or more. Film-like graphite. An artificial film-like graphite obtained by heat-treating a polyimide film having a thickness of 125 μm or more and a birefringence of 0.12 or more, and having a thermal conductivity of 1486 W / m · K or more. Graphite. An artificial film-like graphite obtained by heat-treating a polyimide film having a birefringence of 0.12 or more , having a thickness of 55 μm or more and a thermal conductivity of 1486 W / m · K or more. Film-like graphite. An artificial film-like graphite obtained by heat-treating a polyimide film having a birefringence of 0.12 or more, having a thickness of 32.5 μm or more, and a thermal conductivity of 1536 W / m · K or more. Film-like graphite characterized by

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